Modem integrated circuits (ICs) operate according to strictly defined electrical parameters. In order to stay within those parameters, ICs need an accurate and responsive source of power in the form of device power supplies. These electronic assemblies typically implement control circuitry to minimize changes in the output due to a variety of factors.
The current demand by an IC often varies between high and low extremes during operation. Under these conditions, it is important that the supply voltage to the IC remain as constant as possible during these current load swings. In order to minimize supply voltage changes, conventional design considerations involve varying two factors. First, a large amount of capacitance is often placed near the IC to supply near instantaneous current, when needed. Second, the device power supply is typically optimized to respond to the change in current demand as rapidly as possible.
For automatic test equipment (ATE) applications, the power supply is typically called a device-under-test (DUT) power supply. In this context, the DUT acts as a load on the tester. For modem DUTs, such as VLSI microprocessors, operation typically demands relatively high currents during a test. The current required by the DUT may vary from very low to very high values during a very short period of time (nanoseconds).
DUT power supply manufacturers often optimize the response of the power supply control loop by sensing the voltage at the DUT and feeding it back to an error amplifier to generate an error signal at the power supply control amplifier. While this technique often optimizes the response to a certain degree, an end user of the power supply often has the freedom to change the amount of capacitance located near the DUT. Moreover, the load itself may have a wide range of selectable current steps. As a result, it is difficult for the power supply manufacturer to predict the optimal control loop performance for all possible combinations.
Typical DUT power supplies often include a main loop amplifier that drives one or more output stages. A control loop is established through sense lines that feed the power supply output back to the error amplifier. The error amplifier compares the actual output to the desired output and provides an error signal to the main loop amplifier to adjust the output accordingly.
Conventional power supply compensation methods to smooth overshoot or undershoot responses typically focus on the main loop amplifier. This has classically been a first order compensation scheme employing a single resistor and capacitor in series between the amplifier output and its negative input. Different compensations to optimize the error signal are achieved by changing the values of the resistor and capacitor. Older conventional schemes typically employed a single compensation setting that often compromised performance. Newer conventional schemes have employed from two to eight selectable settings.
However, for any given conventional compensation, it's only optimized for a given combination of load resistance and capacitance. The load (DUT) produces an error signal with a change in voltage per change in time (dv/dt) that's a direct function of the load current, load capacitance, other parasitics, and power supply response time. The supply response time is included since it responds during the droop time.
As noted above, at any given time, the dv/dt is a function of load current, capacitance current and supply current. Initially the load current is supplied from the capacitance. Over time, the power supply starts to share in this current supply to the load. Eventually, the supply provides all the load current. This is a dynamically changing dv/dt for a given set of load, capacitance and loop compensation. The compensation, however, is not dynamic. It only responds in one way, dictated by the RC values selected.
Another problem with conventional DUT power supplies involves the possibility that an end user may select the improper compensation setting. This may lead to full power oscillation of the power supply outputs, and cause destruction of device-interface-boards (DIBs), VLSI parts, and DIB capacitors.
What is needed and heretofore unavailable is a high-accuracy device power supply for both ATE and non-ATE applications capable of addressing the compensation problem without compromising performance and accuracy. The control loop circuit and method of the present invention satisfies this need.